1. Field of the Invention
The present invention relates to a magnetic toner used for developing an electrostatic image which is formed according to an image forming method such as an electrophotographic method, an electrostatic recording method, a magnetic recording method, or a toner jet recording method.
2. Description of the Related Art
A large number of methods have been conventionally known as electrophotographic methods. A general electrophotographic method involves: forming an electrostatic latent image on an electrostatic image bearing member (hereinafter, referred to as “photosensitive member”) using a photoconductive substance with various means; developing the latent image with toner to obtain a visible image (toner image); transferring the toner image onto a recording medium such as paper as required; and fixing the toner image onto the recording medium by means of heat, pressure, or the like to obtain a copied product.
Of such electrophotographic methods, a jumping development method using magnetic toner has been extensively used as a method with which a high-definition image with little fogging can be obtained. The jumping development method involves: applying a thin layer of insulating magnetic toner to a developer bearing member; frictionally charging the toner; and bringing the toner extremely close to (but not in contact with) an electrostatic latent image to be opposite to the latent image through the action of a magnetic field, to thereby develop the image.
However, a development method using magnetic toner inevitably poses a problem owing to the magnetic toner to be used. The problem is such that the flowability, environmental stability, and frictional chargeability of the toner reduce because a considerable amount of magnetic material in fine powder form is mixed and dispersed in the toner and part of the magnetic material is exposed to the toner particle surface. As a result, during long-term use, the magnetic material peels off the toner particles owing to the rubbing of toner particles with each other or the rubbing between a toner particle and a regulating member. Thus, the toner deteriorates, with the result that image failures such as a reduction in image density and uneven in density called sleeve ghost occur.
Many propositions have been conventionally made on the deterioration of image properties involved in the exposure of a magnetic material in terms of the toner structure.
For example, a special toner has been reported, in which only a specific part inside the particles contains a magnetic material particle. To be concrete, the special toner is a toner for pressure fixation manufactured through 2 to 3 steps including: the step of manufacturing a core particle; the step of allowing a magnetic material to dry-adhere to the core particle; and the step of forming a shell layer after the dry-adhesion, in which the magnetic material is present only in an intermediate layer of the toner particles (see JP 60-003647 A and JP 63-089867 A). A toner has also been reported, which is structured such that a resin layer having no magnetic material particles is formed in the vicinity of the toner particle surface to have a thickness equal to or greater than a predetermined thickness (see JP 07-209904 A).
However, it has been recently found out that a toner of such a form poses several problems in achieving an increase in image quality when the toner has a small average particle diameter, for example, an average particle diameter of 10 μm or less. One of the problems is that charge up easily occurs under a low-temperature and low-humidity environment. Such a toner as one described above in which only a specific part inside the particles contains a magnetic material particle has essentially no magnetic material present on the toner surface, so that the toner surface is composed of a resin. According to the studies made by the inventors of the present invention, the toner particle surface has a high resistance and directly reflects the charging property of the resin. Therefore, charge up is remarkable as the toner particle diameter reduces or the toner specific surface area increases.
In addition, each of JP 2001-312097 A (U.S. Pat. No. 6,447,969) and JP 2002-251037 A (U.S. Pat. No. 6,465,144) describes a magnetic toner manufactured by a polymerization method in which no magnetic material is present on the surface of the toner particles but a magnetic material is present in the vicinity of the toner particle surface. However, neither of those prior arts has made an investigation into embedding of an external additive into toner particles, so that the durability is susceptible to improvement.
In recent years, analog printers and analog copying machines have been gradually replaced with digital printers and digital copying machines. Such printers and copying machines have been strongly required to obtain high-resolution images excellent in latent image reproducibility, and to allow an increase in print speed and a reduction in power consumption.
A printer is taken as an example here. A ratio of power consumption at the fixation step to the total power consumption is considerably large, so that the power consumption increases with increasing fixation temperature. Furthermore, a problem such as curling of printed-out paper occurs with increasing fixation temperature. In view of such a circumstance, a reduction in fixation temperature has been strongly required.
To cope with such requirements, many investigations have been conventionally made into a reduction in fixation temperature of toner. Many propositions have been made on a substance having low softening point to be added to toner. For example, it has been reported that the dispersibility of a magnetic material in toner is increased and, at the same time, the fixability and offset resistance of the toner are improved by means of a special approach in which the magnetic material surface is treated with a substance having low softening point (see JP 09-319137 A, JP 01-259369 A, and JP 01-259372 A).
However, the compatibility between the low-temperature fixability and offset resistance of the toner is still susceptible to improvement even when such a magnetic material is used, specially an improvement in fixability has been insufficient. In particular, when a process speed is high, a time period during which the toner and a fixing unit are in contact with each other at the time of fixation is extremely short, so that the toner receives a limited heat quantity. Consequently, toner to be used in a high-speed printer requires a further reduction in fixation temperature.